qapi: input_type_enum(): fix error message

The enum string is pointed to by 'enum_str' not 'name'. This bug
causes the error message to be:

{ "error": { "class": "InvalidParameter",
             "desc": "Invalid parameter 'null'",
             "data": { "name": "null" } } }

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Reviewed-by: Amos Kong <akong@redhat.com>